The dielectric constant of a material is a fundamental physical property of the material and is important in research and development and for the control of industrial processes. For example, careful control of the amount of moisture (which is related to dielectric constant) in a stream of tobacco moving through a pipe or chute is important because if the percent moisture in the tobacco is not within specified restricted ranges, the tobacco will not "cure" properly. Since the dielectric constants of water and tobacco are quite different (water 80, tobacco 5) a technique to rapidly, conveniently and accurately measure the dielectric constant of a stream of tobacco in a chute or pipe is very useful.
Also, shipping costs of a material, where based on gross weight, may be needlessly increased by extra moisture in the material.
One way of determining dielectric constant involves capacitance measuring, as the capacitance of a capacitor is proportional to a constant determined by the physical dimensions of the electrodes of the capacitor and the distance between the electrodes multiplied by the dielectric constant of the material between the electrodes. For example the capacitance, C, in farads, of a parallel plate capacitor is approximated by the well known equation EQU C=K(eA)/d Equation (1)
where K is a constant, e is the dielectric constant of the material between the plates of the capacitor, A is the area of the plates and d is the distance between the plates. In addition to the area of the electrodes and the distance between them, the "end effects" of the electrodes affect the capacitance.
One technique for determining dielectric constant by capacitance methods is shown in U.S. Pat. No. 3,025,465, assigned to the assignee of the present application. In that patent, a sample of material is placed in a test cell which consists of two plates separated by a certain fixed distance. A first set of capacitance measurements is taken, first with the test cell empty and then with the material in the cell. Then, the spacing between the plates is changed by physically moving one of the plates. A second set of capacitance measurements is taken, first with the cell empty and then with the material in the cell. The dielectric constant of the material is determined from the ratio of the difference between the capacitances measured with the material in the test cell and the difference between the capacitances measured with the test cell empty, that is, filled only with air which has a known dielectric constant.
The difference between the capacitances measured with the test cell empty can also be used to calibrate a measuring instrument. Then, the dielectric constant of an unknown material can be determined using the difference between the capacitances measured with the unknown material in the test cell in conjunction with the calibrated measuring instrument. Further details of such measuring techniques are found in U.S. Pat. No. 3,025,465.
U.S. Pat. No. 3,488,758, assigned to the assignee of the present application, also utilizes a capacitance measuring method for determining dielectric constant. However, in that patent, capacitance is not measured directly. Rather, the plates of the test cell are connected across the terminals of a free-running oscillator. The capacitance across the plates of the cell serves as the capacitance of the oscillator. The frequency of the oscillator is gated to a counter so as to produce an increasing count for a predetermined time interval during which the plates are separated by a first distance. As in U.S. Pat. No. 3,025,465, the distance between the plates of the test cell is then changed by moving one of the plates after the first measurement. This count process is repeated and the residual count remaining on the counter is a count proportional to the change in capacitance as a result of movement of the plates. As described above, the dielectric constant of the material can then be determined using a second set of differential measurements for a material having a known dielectric constant, for example, air.
Both of the above patents have the disadvantage that they require that a plate be moved in carrying out the measurement process. Movement of a plate adds time, complexity and cost to the measurement process and also increases the probability of measurement error because position measurement of the movable plate is not perfectly repeatable.
U.S. Pat. No. 4,555,661, also assigned to the assignee of the present application, utilizes a capacitance measuring method for determining the dielectric constant of liquids, slurries, gases, or solids which can be made to flow, without the necessity of physically moving a plate during the measurement process. As in U.S. Pat. No. 3,488,758, the '661 patent does not measure capacitance directly.
The '661 patent discloses in one embodiment a three electrode device which is placed in a vial containing a material for which the dielectric constant is to be determined. Two of the electrodes are connected across the terminals of a free-running oscillator. The third electrode is located between the first and second electrodes. A switch is connected between the first and third electrodes so that the third electrode is at a floating electrical potential when the switch is open. Closing the switch electrically switches the third electrode from a floating electrical potential to an electrical potential equal to that of the first electrode. The first and second electrodes are connected to a free-running oscillator which is in turn connected to a bi-directional counter. With the switch open, the capacitance for the free-running oscillator is primarily determined by the area of the first and second electrodes, the separation of the electrodes and the dielectric constant of the material between those electrodes. A first count, in an increasing direction, of the frequency of the oscillator is made by the bi-directional counter over a predetermined time period. The switch is then closed, thereby electrically switching the third electrode to the electrical potential of the first electrode, and thus changing the value of the capacitance of the free-running oscillator because the distance between the two "plates" of the capacitor has effectively been decreased. A second count, in a decreasing direction, of the frequency of the oscillator is made by the bi-directional counter, leaving a residual count in the bi-directional counter. The dielectric constant of a first material is determined by obtaining a residual count using the above procedure for a second material having a known dielectric constant. A residual count for the first material is then obtained. Since the ratio of the unknown dielectric constant of the first material is proportional to the ratio of the residual counts for the two materials, the dielectric constant of the unknown material can be readily determined from the measured residual counts.
A further embodiment of the '661 patent utilizes a microprocessor to replace the bi-directional counter and perform the counting function, control the switch and determine the dielectric constant by interpolating from a stored table of residual counts for materials of known dielectric constants.
A further embodiment of the '661 patent utilizes five electrodes in a so-called split stator arrangement, which creates a balanced electric field, and two switches. This five electrode embodiment functions in a similar fashion to the three electrode embodiment to determine dielectric constant.
Often, it is desired to measure the dielectric constant of a material "on-line", i.e., without having to remove a sample of the material from a continuous processing of the material. The apparatus described in U.S. Pat. No. 4,555,661 cannot always be used to determine "on-line" the dielectric constant of a material where the material is moving through the inside of a pipe, chute or the like as part of a process. For example, if the electrode device of the '661 patent is used for an "on-line" measurement of the dielectric constant of a stream of tobacco slowly moving through a pipe or chute, the electrodes, which would have to be placed directly into the tobacco, would significantly interfere with the movement of the stream of tobacco and would most likely cause the stream to stop moving, or "bridge" creating air pockets, thus unacceptably interfering with the process.
Further, the electrode device of the '661 patent cannot accurately measure the dielectric constant of the tobacco, or any other bulk solid or powder which does not flow like a liquid, because the electrodes of the device, when stuck into the tobacco, break up the solid structure of the tobacco, creating unacceptably large air pockets. The dielectric constant measured by the '661 patent apparatus would not accurately reflect the dielectric constant of the tobacco only because a significant part of the measurement would represent the dielectric constant of the air pockets. Because the tobacco, when broken up by the electrodes, would not evenly "flow" around the electrodes, the dielectric constant measurement made by the '661 patent apparatus would not accurately reflect the dielectric constant of the tobacco.
Other examples of bulk solids or powders which do not flow like a liquid, for which the '661 apparatus would not be adequate, are coffee beans, nuts, powdered detergents and certain pharmaceuticals.